Method of separating olefins from mixtures with paraffins

ABSTRACT

A process for the separation or concentration of olefinic hydrocarbons from mixtures of olefinic and paraffinic hydrocarbons uses a polyimide membrane, The process is well suited to separating propylene from propylene/propane mixtures. The novel method the membrane exhibits good resistance to plasticization by hydrocarbon components in the gas mixture under practical industrial process conditions.

This application is a continuation of prior application Ser. No.10/353,210, filed Jan. 27, 2003, now U.S. Pat. No. 7,250,545, which ishereby incorporated by reference herein. Application No. 10/353,210claims benefit of priority of provisional patent application No.60/430,327 filed Dec. 2, 2002.

FIELD OF THE INVENTION

This invention relates to a method of separating or concentratingmixtures of olefins and paraffins using a selectively permeablemembrane. More specifically, it relates to a method of using certainpolyimide membranes to selectively separate olefinic hydrocarbons from agas or liquid mixture of olefinic and paraffinic hydrocarbons such asthose generated by petroleum refining industries, petrochemicalindustries, and the like.

BACKGROUND OF THE INVENTION

Olefins, particularly ethylene and propylene, are important chemicalfeedstocks. Typically they are found in nature or are produced asprimary products or byproducts in mixtures that contain saturatedhydrocarbons and other components. Before the raw olefins can be used,they usually must be separated from these mixtures.

Currently, separation of olefin/paraffin mixtures is usually carried outby distillation. However, the similar volatilities of the componentsmake this process costly and complicated, requiring expensivedistillation columns and energy-intensive processing. Jarvelin reportsthat the fractional distillation of propylene/propane mixtures is themost energy-intensive distillation practiced in the United States (HarriJärvelin and James R. Fair, Adsorptive separation of propylene/propanemixtures, Ind. Eng. Chem. Research 32 (1993) 2201-2207). More energyconserving separation processes are needed.

Membranes have been considered for the separation of olefins fromparaffins as an alternative to distillation. However, the separation isdifficult largely because of the similar molecular sizes of thecomponents. Another difficulty is that the feed stream conditions aretypically close to the gas/liquid phase boundary of the mixture. Also,the membrane must operate in a hydrocarbon environment under conditionsof high pressure and temperature. Such harsh conditions tend toadversely affect the durability and stability of separation performanceof many membrane materials. For example, some contaminants plasticizeselectively permeable membrane materials and can cause loss ofselectivity and/or permeation rate. A membrane with sufficiently higholefin/paraffin selectivity, and sufficient durability in long-termcontact with hydrocarbon streams under high pressure and temperature ishighly desired.

Membrane materials for separating olefinic hydrocarbons from a mixtureof olefinic and saturated hydrocarbons have been reported, but none canbe easily or economically fabricated into membranes that offer theunique combination of high selectivity and durability under industrialprocess conditions.

For example, several inorganic and polymer/inorganic membrane materialswith good propylene/propane selectivity have been studied. See M.Teramoto, H. Matsuyama, T. Yamashiro, Y. Katayama, Separation ofethylene from ethane by supported liquid membranes containing silvernitrate as carrier, J. Chem Eng. Japan 19 (1986) 1, and R. D. Hughes, J.A. Mahoney, E. F. Steigelmann, Olefin separation by facilitatedtransport, in: N. N. Li, J. M. Calo (eds.), Membrane Handbook, VanNostrand, New York, 1992. Such materials are difficult to fabricate intopractical industrial membranes. Liquid facilitated-transport membraneshave been demonstrated to have attractive separation performance in thelab, but have been difficult to scale up, and have exhibited decliningperformance in environments typical of an industrial propylene/propanestream.

Solid polymer-electrolyte facilitated-transport membranes appear moreamenable to fabrication into stable thin film membranes. See Ingo Pinnauand L. G. Toy, Solid polymer electrolyte composite membranes forolefin/paraffin separation, J. Membrane Science, 184 (2001) 39-48. Sucha membrane is exemplified in U.S. Pat. No. 5,670,051 (Pinnau et al,1997) wherein a silver tetrafluoroborate/poly(ethylene oxide) membraneexhibited ethylene/ethane selectivity of greater than 1000. However,these membranes are severely limited by their poor chemical stability inthe olefin/paraffin industrial environment.

Carbon hollow-fiber membranes have shown promise in laboratory tests(“Propylene/Propane Separation”, Product Information from CarbonMembranes, Ltd., Israel), but are vulnerable to degradation caused bycondensable organics present in industrial streams. Moreover, carbonmembranes are brittle and difficult to form into membrane modules ofcommercial relevance.

Membranes based on rubbery polymers typically have olefin/paraffinselectivity too low for an economically useful separation. For example,Tanaka et al. report that the single-gas propylene/propane selectivityis only 1.7 for a polybutadiene membrane at 50° C. (K. Tanaka, A.Taguchi, Jianquiang Hao, H. Kita, K. Okamoto, J. Membrane Science 121(1996) 197-207) and Ito reports a propylene/propane selectivity onlyslightly over 1.0 in silicone rubber at 40° C. (Akira Ito and Sun-TakHwang, J. Applied Polymer Science, 38 (1989) 483-490).

Membranes based on glassy polymers have the potential for providingusefully high olefin/paraffin selectivity because of the preferentialdiffusivity of the olefin, which has smaller molecular size than theparaffin.

Glassy polymers already used in gas separation have generally shown onlymodest olefin/paraffin selectivity. For example, Ito has reported thatfilms of polysulfone, ethyl cellulose, cellulose acetate and cellulosetriacetate exhibit propylene/propane selectivity of 5 or less (Akira Itoand Sun-Tak Hwang, Permeation of propane and propylene throughcellulosic polymer membranes, J. Applied Polymer Science, 38 (1989)483-490).

U.S. Pat. No. 4,623,704 describes a process utilizing a cellulosetriacetate membrane for recovering ethylene from the reactor vent of apolyethylene plant. However, the vent stream that contained 96.5%ethylene is moderately upgraded to only 97.9% in the permeate stream forrecycle to the reactor.

Membrane films of poly(2,6-dimethyl-1,4-phenylene oxide) exhibited puregas propylene/propane selectivity of 9.1 (Ito and Hwang, Ibid.) Higherselectivity has been reported by Ilinitch et al. (J. Membrane Science 98(1995) 287-290, J. Membrane Science 82 (1993) 149-155, and J. MembraneScience 66 (1992) 1-8), but the values at higher pressure were uncertainand were accompanied by undesirable plasticization of the membrane bypropylene.

Polyimide membranes have been studied extensively for the separation ofgases and to some degree for the separation of olefins from paraffins.Lee et al. (Kwang-Rae Lee and Sun-Tak Hwang, Separation of propylene andpropane by polyimide hollow-fiber membrane module, J. Membrane Science73 (1992) 37-45) disclose a hollow fiber membrane of a polyimide thatexhibited mixed-gas propylene/propane selectivity in the range of 5-8with low feed pressure (2-4 barg). The composition of the polyimide wasnot disclosed.

Krol et al. (J. J. Krol, M. Boerrigter, G. H. Koops, Polyimide hollowfiber gas separation membranes: preparation and the suppression ofplasticization in propane/propylene environments, J. Membrane Science.184 (2001) 275-286) report a hollow fiber membrane of a polyimidecomposed of biphenyltetracarboxylic dianhydride and diaminophenylindanewhich exhibited a pure-gas propylene/propane selectivity of 12; however,the membrane was undesirably plasticized by propylene at propylenepressure as low as 1 barg.

Polyimides based on 4,4′-(hexafluoroisopropylidene)diphthalic anhydride(6FDA) and aromatic diamines have been found to provide a favorablecombination of propylene permeability and propylene/propane selectivity.Permeation data for dense-film membranes of two different6FDA-containing polyimides have been reported to have pure gasselectivity for propylene/propane in the range of 6-27. (C.Staudt-Bickel et al, Olefin/paraffin gas separations with 6FDA-basedpolyimide membranes, J. Membrane Science 170 (2000) 205-214). Higherselectivity for similar 6FDA polyimides has been reported in U.S. Pat.No. 5,749,943 (Shimazu et al); however, it is anticipated that mixed-gasselectivity at high pressure will be much lower due to plasticization bythe propylene-rich feed gas.

U.S. Pat. Nos. 4,532,041; 4,571,444; 4,606,903; 4,836,927; 5,133,867;6,180,008; and 6,187,987 disclose membranes based on a polyimidecopolymer derived from the co-condensation of benzophenone3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and a mixture ofdi(4-aminophenyl)methane and a mixture of toluene diamines useful forliquid separations.

U.S. Pat. Nos. 5,605,627; 5,683,584; and 5,762,798 disclose asymmetric,microporous membranes based on a polyimide copolymer derived from theco-condensation of benzophenone-3,3′,4,4′-tetracarboxylic aciddianhydride (BTDA) and a mixture of di(4-aminophenyl)methane and amixture of toluene diamines useful for liquid filtration or dialysismembranes.

U.S. Pat. No. 5,635,067 discloses a fluid separation membrane based onblends of phenylindane-containing polyimide polymers with polyimidesderived from the condensation of benzophenone-3,3′,4,4,′-tetracarboxylicacid dianhydride (BTDA) with toluenediisocyanate (TDI) and4,4′-methylene bisphenylisocyanate (MDI) and/or polyimides derived fromthe condensation of BTDA and pyromellitic dianhydride with TDI and MDI.

A significant shortcoming of published data for the separation ofolefins from paraffins using membranes is the absence of data underpractical industrial conditions: e.g., high feed and permeate pressureand high temperature. These are conditions under which plasticization ofthe membrane material could become significant and which could result insubstantial decline in membrane performance over extended periods oftime. In spite of the considerable efforts to provide industriallyviable membranes for the separation of olefins from paraffins, none hasproven to meet the performance criteria required for industrialapplication.

SUMMARY OF THE INVENTION

The invention is directed to a membrane separation process forseparating an olefin from a mixture of olefins and paraffins comprising:

(a) providing a two-sided, selectively permeable membrane comprising apolymer or copolymer having repeating units of formula (I):

in which R₂ is a moiety of composition selected from the group ofconsisting of formula (A), formula (B), formula (C) and a mixturethereof,

Z is a moiety of composition selected from the group consisting offormula (L), formula (M), formula (N) and a mixture thereof; and

R₁ is a moiety of composition selected from the group consisting offormula (Q), formula (T), formula (S), and a mixture thereof,

(b) contacting one side of the membrane with a feed mixture comprisingan olefin compound and a paraffin compound having a number of carbonatoms at least as great as the olefin compound,

(c) causing the feed mixture to selectively permeate through themembrane, thereby forming on the second side of the membrane anolefin-enriched permeate composition which has a concentration of theolefin compound greater than that of the feed mixture,

(d) removing from the second side of the membrane the olefin-enrichedpermeate composition, and

(e) withdrawing from the one side of the membrane an olefin-depletedcomposition.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a method of selectively separatingolefinic hydrocarbons from paraffinic hydrocarbons using a membranecontaining certain polyimide polymers, copolymers and blends thereof.The polymers which form these polyimides have repeating units as shownin the following formula (I):

in which R₂ is a moiety of composition selected from the group ofconsisting of formula (A), formula (B), formula (C) and a mixturethereof,

Z is a moiety of composition selected from the group consisting offormula (L), formula (M), formula (N) and a mixture thereof; and

R₁ is a moiety of composition selected from the group consisting offormula (Q), formula (T), formula (S), and a mixture thereof,

In a preferred embodiment the polyimide that forms the selective layerof the membrane has repeating units as shown in the following formula(II):

In this embodiment, moiety R₁ is of formula (Q) in 0-100% of therepeating units, of formula (T) in 0-100% of the repeating units, and offormula (S) in a complementary amount totaling 100% of the repeatingunits. A polymer of this structure is available from HP Polymer GmbHunder the tradename P84 and is much preferred for use in the presentinvention. P84 is believed to have repeating units according to formula(II) in which R₁ is formula (Q) in about 16% of the repeating units,formula (T) in about 64% of the repeating units and formula (S) in about20% of the repeating units. P84 is believed to be derived from thecondensation reaction of benzophenone tetracarboxylic dianhydride (BTDA,100 mole %) with a mixture of 2,4-toluene diisocyanate (2,4-TDI, 64 mole%), 2,6-toluene diisocyanate (2,6-TDI, 16 mole %) and4,4′-methylene-bis(phenylisocyanate) (MDI, 20 mole %).

In another preferred embodiment, the polyimide that forms the selectivelayer has repeating units of compositions selected from among thoseshown in the following formulas (IIIa and IIIb):

The repeating units can be exclusively of formula (IIIa) or formula(IIIb). Preferably, the repeating units are a mixture of formulas (IIIa)and (IIIb). In these embodiments, moiety R₁ is a composition of formula(Q) in about 1-99% of the repeating units, and of formula (T) in acomplementary amount totaling 100% of the repeating units, and a is inthe range of about 1-99% of the total of a and b.

A preferred polymer of this structure is available from HP Polymer GmbHunder the tradename P84-HT325. P84-HT325 is believed to have repeatingunits according to formulas (IIIa and IIIb) in which the moiety R₁ is acomposition of formula (Q) in about 20% of the repeating units and offormula (T) in about 80% of the repeating units, and in which a is about40% of the total of a and b. P84-HT325 is believed to be derived fromthe condensation reaction of benzophenone tetracarboxylic dianhydride(BTDA, 60 mole %) and pyromellitic dianhydride (PMDA, 40 mole %) with2,4-toluene diisocyanate (2,4-TDI, 80 mole %) and 2,6-toluenediisocyanate (2,6-TDI, 20 mole %).

In yet another preferred embodiment, the selectively permeable portionof the membrane can be formed of a material comprising a blend of theabove mentioned polymers. For example, it is contemplated that amembrane can be formed from a blend comprising a first polymer havingrepeating units of formula (IIIa), formula (IIIb) as defined above, or amixture of formulas (IIIa) and (IIIb) and a second polymer havingrepeating units of formula (II) as defined above. Greater preference isgiven to a membrane of a blend consisting essentially of the first andsecond polymers. In such preferred composition, the second polymershould constitute about 10-90 wt. % of the total of the first polymerand the second polymer.

The polyimides should be of suitable molecular weight to be film formingand pliable so as to be capable of being formed into continuous films ormembranes. The polyimides of this invention preferably have a weightaverage molecular weight within the range of about 20,000 to about400,000 and more preferably about 50,000 to about 300,000. The polymercan be formed into films or membranes by any of the diverse techniquesknown in the art. The polymers are usually glassy and rigid, andtherefore, may be used to form a single-layer membrane of an unsupportedfilm or fiber of the polymer. Such single-layer films are normally toothick to yield commercially acceptable transmembrane flux of thepreferentially permeable component of the feed mixture. To be moreeconomically practical, the separation membrane can comprise a very thinselective layer that forms part of a thicker structure. This structuremay be, for example, an asymmetric membrane, which comprises a thin,dense skin of selectively permeable polymer and a thicker micro-poroussupport layer which is adjacent to and integrated with the skin. Suchmembranes are described, for example, in U.S. Pat. No. 5,015,270 toEkiner.

In a preferred embodiment, the membrane can be a composite membrane,that is, a membrane having multiple layers of typically differentcompositions. Modern composite membranes typically comprise a porous andnon-selective support layer. It primarily provides mechanical strengthto the composite. A selective layer of another material that isselectively permeable, is placed coextensively on the support layer. Theselective layer is primarily responsible for the separation properties.Typically, the support layer of such a composite membrane is made bysolution-casting a film or spinning a hollow fiber. Then the selectivelayer is usually solution coated on the support in a separate step.Alternatively, hollow-fiber composite membranes can be made byco-extrusion of both the support material and the separating layersimultaneously as described in U.S. Pat. No. 5,085,676 to Ekiner.

The membranes of the invention may be housed in any convenient type ofseparation unit. For example, flat-sheet membranes can be stacked inplate-and-frame modules or wound in spiral-wound modules. Hollow-fibermembranes are typically potted with a thermoset resin in cylindricalhousings. The final membrane separation unit can comprise one or moremembrane modules. These can be housed individually in pressure vesselsor multiple modules can be mounted together in a common housing ofappropriate diameter and length.

In operation, a mixture of one or more olefin compounds and one or moreparaffin compounds is contacted with one side of the membrane. Under asuitable driving force for permeation, such as imposing a pressuredifference between the feed and permeate sides of the membrane, theolefin compounds pass to the permeate side at higher rate than theparaffin compounds of the same number of carbon atoms. That is, a threecarbon olefin permeates faster than a three carbon paraffin. Thisproduces an olefin-enriched stream which is withdrawn from the permeateside of the membrane. The olefin-depleted residue, occasionally referredto as the “retentate”, is withdrawn from the feed side.

The novel process can operate under a wide range of conditions and isthus adapted to accept a feed stream supplied from diverse sources. Ifthe feed stream is a gas that exists already at a sufficiently high,above-atmospheric pressure and a pressure gradient is maintained acrossthe membrane, the driving force for separation can be adequate withoutraising feed stream pressure farther. Otherwise, the feed stream can becompressed to a higher pressure and/or a vacuum can be drawn on thepermeate side of the membrane to provide adequate driving force.Preferably the driving force for separation should be a pressuregradient across the membrane of about 0.7 to about 11.2 MPa (100-1600psi).

The novel process can accept a feed stream in either the gaseous stateor the liquid state. The state of matter will depend on the compositionand on the pressure and temperature of the olefin/paraffin feed stream.When the feed stream is in the liquid state, the separation can becarried out by the pervaporation mechanism. Basically, in pervaporation,components of the liquid feed mixture in contact with the membranepermeate and evaporate through the membrane, thereby separating thecomponent in the vapor phase.

This invention is particularly useful for separating propylene frompropylene/propane mixtures. Such mixtures are produced as effluentstreams of olefin manufacturing operations, and in various processstreams of petrochemical plants, for example. Thus in a preferredembodiment, the process involves passing a stream comprising propyleneand propane in contact with the feed side of a membrane that isselectively permeable with respect to propylene and propane. Thepropylene is concentrated in the permeate stream and the retentatestream is thus correspondingly depleted of propylene. The membranes ofthis invention exhibit unexpectedly high propylene/propane selectivitywhich distinguishes them from prior art membranes. Furthermore, themembranes of this invention exhibit stable performance over long periodsof time under conditions where membranes of the prior art degradesignificantly in performance.

The fundamental steps of the separation process include

contacting one side of the membrane with a feed mixture comprising anolefin compound and a paraffin compound having a number of carbon atomsat least as great as the olefin compound,

causing the feed mixture to selectively permeate through the membrane,thereby forming on the second side of the membrane an olefin-enrichedpermeate composition which has a concentration of the olefin compoundgreater than that of the feed mixture,

removing from the second side of the membrane the olefin-enrichedpermeate composition, and

withdrawing from the one side of the membrane an olefin-depletedcomposition which has a concentration of the olefin compound less thanthat of the feed mixture.

This invention is now illustrated by examples of certain representativeembodiments thereof, wherein all parts, proportions and percentages areby weight unless otherwise indicated. All units of weight and measurenot originally obtained in ST units have been converted to SI units. Theentire disclosures of U.S. Patents named in the following examples arehereby incorporated by reference herein.

EXAMPLES Example 1 Propylene/Propane Gas Separation with P84 Membrane

Asymmetric hollow-fiber membrane of P84 was spun from a solution of 32%P84, 9.6% tetramethylenesulfone and 1.6% acetic anhydride inN-methylpyrrolidinone (NMP) with methods and equipment as described inU.S. Pat. Nos. 5,034,024 and 5,015,270. The nascent filament wasextruded at a rate of 180 cm³/hr through a spinneret with fiber channeldimensions of outer diameter 559 μm and inner diameter equal to 254 μmat 75° C. A fluid containing 85% NMP in water was injected into the boreof the fiber at a rate of 33 cm³/hr. The nascent fiber traveled throughan air gap of 5 cm at room temperature into a water coagulant bath at24° C. and the fiber was wound up at a rate of 52 m/min.

The water-wet fiber was washed with running water at 50° C. to removeresidual solvent for about 12 hours and then sequentially exchanged withmethanol and hexane as taught in U.S. Pat. Nos. 4,080,744 and 4,120,098,followed by vacuum drying at room temperature for 30 minutes. After thatthe fibers were dried at 100° C. for one hour. Samples of fiber wereformed into four test membrane modules of 52 fibers each. The fiber inthe modules was treated to seal defects in the separating layer with amethod similar to the method described in U.S Pat. No. 4,230,463. Thefiber was thus contacted with a solution of 2% wt. 1-2577 Low-VOCConformal Coating (Dow Corning Corporation) in 2,2,4-trimethylpentanefor 30 minutes and then dried.

The modules were measured in permeation of a feed of mixedpropylene/propane (50:50 mole %). The feed mixture was provided in thevapor state by controlling the feed pressure at 2.8 MPa (400 psig) andthe feed temperature at 90° C. The feed mixture was supplied to contactthe outside of the fibers and the permeate stream was collected atatmospheric pressure. The permeate flowrate was measured by volumetricdisplacement with bubble flowmeters. The feed flowrate was maintained atgreater than twenty times the permeate flowrate. This rate was highenough that the composition on the feed side remained roughly constantwhile the feed mixture permeated the membrane. This was done to simplifycalculation of the membrane permeation performance. The composition ofthe permeate stream was measured by gas chromatography with a flameionization detector. The average permeate composition was 92.2%propylene and 7.8% propane.

The performance of the membrane was expressed in terms of propylenepermeance and propylene/propane selectivity. The permeance is theflowrate of propylene across the membrane normalized by the membranesurface area and the propylene partial pressure difference across themembrane. It is reported in gas permeation units (“GPU”). One GPU equals10⁻⁶ cm³(at standard temperature and pressure “STP”)/(sec·cm²·cmHg). Thepropylene/propane selectivity is the ratio of the permeance of propylenedivided by the permeance of propane. The performance of the four modulesis shown in Table 1.

TABLE I Propylene Permeance (1) Propylene/Propane GPU selectivity (1)1.3 12.0 0.97 12.5 1.4 12.9 1.3 13.1 (1) measured after 24 hours

Example 2 Propylene/Propane Gas Separation with P84 Non-posttreatedMembrane

A sample of the fiber from Example 1 was processed and formed into atest module as in Example 1 except that the fiber was not treated toseal defects in the separating layer. The propylene permeance was 1.7GPU and the propylene/propane selectivity was 7.5. Although theselectivity was lower than the selectivity of the treated fiber ofExample 1, it was high enough to suggest that the P84 fiber withacceptable performance characteristics can be produced as an asymmetricmembrane without the sealing posttreatment.

Example 3 Propylene/Propane Gas Separation with P84 Membrane

Asymmetric hollow-fiber membrane of P84 was prepared as in Example 1with the following two changes: (a) the water-bath temperature waslowered to 8° C. and (b) the spinneret temperature was increased to 87°C. The fiber was washed, dried and built into test modules and tested inpermeation of a 50:50 mole % mixed propylene/propane feed mixture as inExample 1. The propylene permeance was 0.61 GPU and thepropylene/propane selectivity was 15.

Example 4 Durability of P84 Membrane in Propylene/Propane Gas Separationwith P84 Membrane

Asymmetric hollow-fiber membrane of P84 similar to the fiber of Example3 was tested for duration of 4 days at 90° C. with a 50:50 mole % feedmixture of propylene/propane at 2.8 MPa (400 psig). The test wasdesigned to simulate commercial operating conditions. Results are shownin Table II. No decline in selectivity was observed. A slight declinewas observed in propylene permeance, which stabilized after the secondday.

TABLE II Feed Propylene/ Propylene Pressure Propane permeance Time MPa(psig) Selectivity GPU 4 hours 1.7 (250) 13 0.76 1 day 1.7 (250) 13 0.962 days 1.7 (250) 13 0.73 3 days 2.8 (400) 12 0.61 4 days 2.8 (400) 140.61

Example 5 Propylene/Propane Liquid Feed Separation with P84 Membrane

One of the modules of Example 1 was tested using a 50:50 mole % feedmixture of propylene/propane. Feed pressure and temperature werecontrolled at 2.8 MPa (400 psig) and 50° C., respectively, to place thefeed mixture in the liquid state. The permeate was withdrawn atatmospheric pressure, therefore the permeate was in the vapor phase. Forthis type of separation the concentration difference across the membraneis usually considered to be the driving force for separation instead ofthe partial pressure difference as used in gas or vapor permeation. Forcomparison of the results of this Example with permeation under vaporstate feed conditions, the simplifying mathematical treatment describedin J. G. Wijmans and R. W. Baker, A simple predictive treatment of thepermeation process in pervaporation, J. Membrane Science 79 (1993)101-113) was applied. Such analysis assumes that the liquid feedevaporates to produce a saturated vapor phase on the feed side of themembrane and then permeates through the membrane driven by a partialpressure gradient. This analysis provides a mathematical model thatincludes terms for feed-side and permeate-side vapor pressures andpermeance and selectivity comparable to those used in the separation ofgaseous state feed mixtures. The model also contains a term related tothe liquid-vapor equilibrium. With the feed mixture of 50:50 mole %propylene/propane in the liquid state, the membrane produced a permeatestream of 93% propylene. By application of the model, it was determinedthat the propylene permeance was 0.46 GPU and the propylene/propaneselectivity was 16. In separate testing with feed mixture of the samecomposition in the vapor state at 2.8 MPa (400 psig) and 90° C., thepropylene permeance was 0.95 GPU and the propylene/propane selectivitywas 13. This shows that the membrane of P84 can be useful for separationservice for liquid propylene/propane.

Example 6 Propylene/Propane Gas Separation with a Membrane of P84Blended with P84-HT325

Asymmetric hollow-fiber membrane of a 1:1 blend of P84 and P84-HT325 wasspun from a solution of 16% P84, 16% P84-HT325, 9.6% tetramethylenesulfone and 1.6% acetic anhydride in NMP by the process described inExample 1. The spinning conditions and equipment were similar exceptthat the spinneret temperature was 85° C., the bath temperature was 8°C. and the air gap was 10 cm. The fiber was formed into a module whichwas tested for permeation of a propylene/propane (50:50 mole %) feedmixture as in Example 1. The permeation performance was 1.9 GPUpropylene permeance and 11.9 propylene/propane selectivity.

Example 7 Propylene/Propane Liquid Feed Separation with a Membrane ofP84 Blended with P84-HT325

The module of 1:1 blend of P84 and P84-HT325 of Example 6 was testedwith 50:50 mole % feed mixture of propylene/propane. The feed mixturewas maintained in the liquid state by applying the conditions describedin Example 5, i.e., the feed pressure was 2.8 MPa (400 psig) and thetemperature was 50° C. The permeate was withdrawn as a vapor atatmospheric pressure.

The membrane produced a permeate with 93.6% propylene; the propylenepermeance was 0.6 GPU and the propylene/propane selectivity was 15.5.This shows that the membrane of 1:1 blend of P84 and P84-HT325 canprovide useful separation with liquid propylene/propane feed.

Example 8 Propylene/Propane Liquid Feed Separation with a Membrane ofP84 Blended with P84-HT325

The test in Example 7 (i.e., with membrane of 1:1 blend of P84 andP84-HT325) was continued for a duration of 100 hours, to assess membraneperformance stability under simulated commercial conditions. Results areshown in Table III. No significant decline was observed.

TABLE III Propylene Time Propylene/Propane Permeance Hours SelectivityGPU 24 15.5 0.56 GPU 60 15.9 0.59 GPU 84 15.6 0.67 GPU 110 15.8 0.67 GPU

Example 9 Propylene/Propane Gas Separation with P84 Dense Film Membrane

A thin dense film of P84 polymer was cast from a solution comprising 20%P84 in NMP. The film was dried at 200° C. in a vacuum oven for fourdays. A sample of the polymer film was tested in a modified 47-mmultrafiltration style permeation cell (Millipore), using a feed mixtureof 50:50 mole % propylene/propane at 2.8 MPa (400 psig) pressure and 90°C. temperature. The permeate pressure was 2-5 mm Hg. The feed flowratewas high enough to ensure low conversion of the feed into permeate sothat the composition on the feed side was constant. The compositions ofthe feed and permeate streams were measured by gas chromatography with aflame ionization detector. The permeate flowrate was determined from theincrease in pressure over time in the fixed-volume permeate chamber ofthe permeation cell.

The permeation performance of the polymer is characterized by the twoparameters: propylene permeability and propylene/propanepermselectivity. The permeability is the flowrate of propylene acrossthe film normalized by the film surface area and film thickness and bythe propylene partial pressure difference across the film. Units ofpermeability are Barrers. One Barrer equals 10⁻¹⁰cm³(STP)·cm/(sec·cm²·cm Hg). The propylene/propane permselectivity isthe ratio of the propylene and propane permeabilities. The propylenepermeability of the P84 film at 90° C. and 2.8 MPa (400 psig) was 0.24Barrers; and the propylene/propane permselectivity was 15.5. Thepermselectivity was in good agreement with the selectivity measured withhollow-fiber membranes of P84 polymer.

Example 10 Propylene/Propane Separation with a Membrane ofTDI+BTDA:BPDA(1:1)

A dense film of a copolymer of toluenediisocyanate (TDI, a mixture of20% 2,6-toluenediisocyanate and 80% 2,4-toluenediisocyanate) and a 1:1mixture of benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride(BTDA) with 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) wastested in permeation with 50:50 mole % mixed propylene/propane feed at2.8 MPa (400 psig) and 90° C. as in Example 9. The propylenepermeability of the film was 0.48 Barrers and the propylene/propanepermselectivity was over 16.

Comparative Example 1 Polypropylene/propane Separation with aTraditional Composition Fiber Membrane

Samples of composite hollow-fiber membrane of Matrimid® 5218 a copolymerof 5,x-amino-(4-aminophenyl)-1,1,3 trimethyl indane and3,3′,4,4′-benzophenone tetracarboxylicdianhydride (Vantico, Inc.) weretested in permeation over a 72-hour period with a feed mixture of 50:50mole % propylene/propane at 1.7 MPa (250 psig) and 90° C. as inExample 1. The purpose of the test was to determine the membraneperformance stability under simulated commercial conditions. Thismembrane, described in U.S. Pat. No. 5,468,430 is a commercialgas-separation membrane produced by MEDAL, LP. Results of the test areshown in Table IV.

TABLE IV Time Propylene/Propane Propylene permeance hours SelectivityGPU 2 5.5 9.0 24 7.0 4.8 48 7.1 4.0 72 7.2 3.8

As apparent from these results, the membrane exhibited low selectivityand lost greater than 50% of its initial permeance during the test,unlike the membranes of this invention.

Comparative Example 2 Propylene/Propane Separation with a PolyaramidMembrane

Samples of asymmetric hollow-fiber membrane made from a blend of twoaromatic polyamides were tested in permeation of a feed mixture of 50:50mole % propylene/propane at 2.8 MPa (400 psig) and 90° C. as inExample 1. This membrane is described in U.S. Pat. No. 5,085,774(Example 15). The fiber was spun at a draw ratio of 7.3. It is anestablished gas-separation membrane applied in the separation ofhydrogen from mixtures with hydrocarbons or carbon monoxide. Itexhibited a propylene permeance of 0.23 GPU and a propylene/propaneselectivity of 9.5. This performance was less than that of the novelmembranes having composition of formula (I). This result was unexpectedbecause the membrane of aromatic polyamide has very high selectivity inseparations of other mixtures, for example a selectivity of higher than200 for H₂/CH₄ at 90° C.

Although specific forms of the invention have been selected forillustration in the preceding description which is drawn in specificterms for the purpose of describing these forms of the invention fullyand amply for one of average skill in the pertinent art, it should beunderstood that various substitutions and modifications which bringabout substantially equivalent or superior results and/or performanceare deemed to be within the scope and spirit of the following claims.

1. A membrane separation process for separating an olefin from a mixtureof olefins and paraffins comprising: (a) providing a two-sided,selectively permeable membrane comprising a polymer or copolymer havingrepeating units of formula (I):

in which R₂ is a moiety of composition selected from the group ofconsisting of formula (A), formula (B), formula (C) and a mixturethereof,

Z is a moiety of composition selected from the group consisting offormula (L), formula (M), formula (N) and a mixture thereof; and

R₁ is a moiety of composition selected from the group consisting offormula (Q), formula (T), formula (S), and a mixture thereof,

(b) contacting one side of the membrane with a feed mixture comprisingan olefin compound and a paraffin compound having a number of carbonatoms at least as great as the olefin compound, (c) causing the feedmixture to selectively permeate through the membrane, thereby forming onthe second side of the membrane an olefin-enriched permeate compositionwhich has a concentration of the olefin compound greater than that ofthe feed mixture, (d) removing from the second side of the membrane theolefin-enriched permeate composition, and (e) withdrawing from the oneside of the membrane an olefin-depleted composition.
 2. The process ofclaim 1 in which the repeating units are of formula (II)

and in which moiety R₁ is of formula (Q) in 0-100% of the repeatingunits, of formula (T) in 0-100% of the repeating units, and of formula(S) in a complementary amount totaling 100% of the repeating units. 3.The process of claim 1 wherein the feed mixture comprises ethylene andethane.
 4. The process of claim 1, wherein the feed mixture comprisespropylene and propane.
 5. The process of claim 1 further comprising thestep of executing steps (a)-(d) continuously for a duration after aninitial time at which the feed mixture first contacts the membrane, andin which the membrane exhibits a permeance for the olefin compound andthe permeance at 72 hours of executing steps (a)-(d) continuously is atleast 60% of the permeance at the initial time.
 6. The process of claim1 in which the membrane provides a selectivity of the olefin compoundrelative to the paraffin compound of at least
 10. 7. A membraneseparation process for separating an olefin from a mixture of olefinsand paraffins comprising: (a) providing a two-sided, selectivelypermeable membrane comprising a blend of a first polymer and a secondpolymer, (b) contacting one side of the membrane with a feed mixturecomprising an olefin compound and a paraffin compound having a number ofcarbon atoms at least as great as the olefin compound, (c) causing thefeed mixture to selectively permeate through the membrane, therebyforming on the second side of the membrane an olefin-enriched permeatecomposition which has a concentration of the olefin compound greaterthan that of the feed mixture, (d) removing from the second side of themembrane the olefin-enriched permeate composition, and (e) withdrawingfrom the one side of the membrane an olefin-depleted composition, and inwhich the first polymer comprises repeating units of moieties selectedfrom the group consisting of formula (IIIa) and formula (IIIb),

where R₁ is a moiety of composition selected from the group consistingof formula (Q), formula (T) and a mixture thereof,

where R₁ is formula (Q) in about 1-99% of the repeating units of thefirst polymer, R₁ is formula (T) in a complementary amount totaling 100%of the repeating units of the first polymer, and a is in the range ofabout 1-99% of a+b, and in which the second polymer comprises repeatingunits of formula (IV)

where R₃ is formula (Q) in 0-100% of the repeating units of the secondpolymer, formula (T) in 0-100% of the repeating units of the secondpolymer, and formula (S)

in a complementary amount totaling 100% of the repeating units of thesecond polymer.
 8. The process of claim 7 in which the second polymerconstitutes about 10-90 wt. % of the blend of the polymer and the secondpolymer.
 9. The process of claim 8 wherein the feed mixture is in theliquid state.
 10. The process of claim 9 in which the membrane providesa permeance of the olefin compound of at least about 0.4 GPU.